The procedure for preparing the above-described new zeolite material is based on the combined use of ion exchanges with ammonium cations (or optionally protons), of thermal treatments in the optional presence of steam and of acid etchings. These various treatments, which can be considered as unitary steps, have already been used in the prior art for decationizing, dealuminating and stabilizing zeolites synthesized with low SiO.sub.2 /Al.sub.2 O.sub.3 ratios, as Y zeolite (U.S. Pat. Nos. 3,595,611, 3,966,882), offretite (French patent No. 2 569 678) and omega zeolite (French patent No. 2 583 654). However, the state of the art shows that the above-mentioned treatments were never used successfully to prepare a decationized, dealuminated and stabilized L zeolite conforming with the present invention. More precisely, to the extent of our knowledge, such a solid was not possible to prepare by any method whatsoever. As a matter of fact, L zeolite is a structure considered as very unstable both thermally and with respect to the acid treatments. As shown hereinafter, most of the authors agree to conclude that even a partial removal of the potassium cations initially contained therein results, irreparably, in a very severe degradation or even in a destruction of the crystalline frame during the conventional stabilizing treatments (roasting under steam for example). The decationizing, dealuminating and stabilizing treatments according to the invention, when used in adequate manner, give a zeolite of L structure, of reduced or, if necessary, very reduced content of alkalis (potassium and sodium), dealuminated and thermally very stable. In addition to the considerable improvement of the thermal stability of the solid, the treatments described according to the invention lead, through removal of alkali cations and control of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio, to L zeolites whose acid properties are clearly much higher than those of the starting zeolite and than those of L zeolites modified according to the prior art procedures. Solids of L structure of the present invention are suitable as catalysts or catalyst carriers for use in reactions involving an acid mechanism, such for example as cracking, hydrocracking or hydroisomerization of oil cuts.
L zeolite is a synthetic zeolite, without any known natural equivalent, which was discovered in 1958 (U.S. Pat. Nos. 2,711,565, 3,216,789). It is synthesized in the presence of potassium cations and optionally of sodium cations. The chemical composition of L zeolite in hydrated form ( R. BARRER and H. VILLIGER, Z. KRISTALLOGR, Bd 128, 3-6, p. 352) is typically: (K.sub.0.91 Na.sub.0.08).sub.2 Al.sub.2 O.sub.3 6.05 SiO.sub.2.
The SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio is generally liable to vary within the range of 5.2 to 6.9 and the molar (K/K+Na) in the range from 0.3 to 1.0 (U.S. Pat. Nos. 2,711,565, 3,216,789). Most of the L zeolites which have been the object of surveys published in the scientific literature, have a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio very close to 6 and have potassium as cation in a highly major proportion. The crystallographic structure of L zeolite has been determined in 1969 (R. BARRER and H. VILLIGER, Z. KRISTALLOGR, Bd 128, 3-6, p. 352). It crystallizes in hexagonal system with the following mesh parameters: EQU a=1.84.+-.0.004 nm c=0.752.+-.0.003 nm.
The crystalline frame is composed of cancrinite cages and hexagonal prisms arrangements which provide in the structure a lattice of unidimensional channels, parallel to c axis, having a twelve oxygen atoms opening and whose diameter is close to 0.71 nm (R. BARRER and H. VILLIGER, Z. KRISTALLOGR, Bd 128, 3-6, p. 352). By its pore size, L zeolite is classified in the category of Y zeolites. L zeolite comprise four types of cation sites (R. BARRER and H. VILLIGER, Z. KRISTALLOGR, Bd 128, 3-6, p. 352) of different accessibility. The moxt inaccessible sites are probably those located in the hexagonal prisms. The existence of such sites, not or hardly accessible, is very probably, as shown hereinafter, the reason of the impossibility to remove all the alkali cations from L zeolite by conventional techniques.
Although L zeolite appears, by its pore structure, as particularly attractive for catalysis, it has been the object of a limited number of surveys and of a still more limited number of applications. The most important application certainly concerns the use of non dealuminated and non decationized KL zeolite containing small particles of noble metal as metallic monofunctional catalyst in catalytic reforming (U.S. Pat. No. 4,104,320, EP No. 145 289, U.S. Pat. No. 4,443,326). The performance of catalysts of high L zeolite content have been investigated to a very small extent and with catalysts not previously subjected to dealumination or stabilization.
Very likely, the lack of interest for L zeolite in acid or acid-metallic bifunctional catalysis results from the checks met in the preparation of dealuminated, stabilized L zeolite of substantially decreased alkali, particularly potassium, cation content. As a matter of fact, it is known in the prior art that potassium cations are not completely removable by conventional ion exchanges (T. WEEKS and A. BOLTON, J. Phys. Chem. 79, (1975), 1924) and that the NH.sub.4 KL form is thermally very unstable (T. WEEKS and A. BOLTON, J. Phys. Chem. 79, (1975), 1924).
The conventional cation exchanges performed with ammonium salt solutions do not provide for the removal of more than about 80% of the potassium cations (T. WEEKS and A. BOLTON, J. Phys. Chem. 79, (1975), 1924).
It is assumed that the potassium cations located in the sites of difficult access, or inaccessible as the hexagonal prisms and the cancrinite cages, are those responsible for the limitations observed for the exchange rates.
The partial removal of K.sup.+ cations may be achieved by treatments in acid medium of KL zeolite or of a L zeolite previously exchanged with NH.sub.4.sup.+. When the treatment conditions are moderate (low acid concentration, low temperature) the crystalline structure of the zeolite is moderately affected but the exchange rates are then limited. A rather low potassium content of 1.56% by weight has been obtained by a direct acid treatment in 0.02 N HCL (N BURSIAN, Y. SHAVANDIN, V. NIKOLINA, L. KIRKACH and Z. DAVYDOVA, Zh. Prik. Khim, vol. 48, n.degree. 3, (1975), 554). However, here L zeolite has very probably a very low SiO.sub.2 /Al.sub.2 O.sub.3 ratio, of about 5; the potassium content of 1.56% by weight then corresponds to an exchange rate of 80-90%. More severe treatment conditions may provide for an increased exchange rate but then lead to a severe destruction of the crystalline structure and to an amorphization of the zeolite. On the other hand, direct treatments in acid solutions do not lead exclusively to a decationization but also to a dealumination of the solid. It is well known in the prior art that a direct dealumination, in solution, of zeolites of high aluminum content, results in the appearance of defects of the structure and in the destruction of the crystalline structure. It is thus not surprising that the decationization tests by direct treatment in acid medium of L zeolite, in imperfectly controlled conditions, were proved to be negative.
As concerns the thermal stability of NH.sub.4 KL zeolite, only solids containing high potassium amounts were surveyed (exchange rate generally lower than 80%). In this respect, the results given in the literature are conclusive: NH.sub.4 KL zeolites are thermally very unstable. Thus NH.sub.4 KL zeolites, exchanged at about 80%, treated with steam, have their crystallinity rate, as measured by X-ray diffraction, very strongly decreased even at a temperature of 550.degree. C., the destruction being effective at 820.degree. C. (M. RUSAK, I. URBANOVICH, N. KOZLOV, Izv. An. BSSR. Ser. Khim, 3. (1976) 37). The same phenomenon is observed during roasting steps conducted under controlled conditions in the absence of steam, for highly exchanged (about 80%) NH.sub.4 KL forms: from 500.degree. C., the crystallinity, measured by X-ray diffraction, is highly degraded (T. WEEKS, and A. BOLTON, J. Phys. Chem, 79, (1975), 1924). According to most of the authors, the unstability of L zeolite in NH.sub.4 KL form during thermal treatments increases with a decreasing potassium content, i.e. when the exchange rate increases (T. KEII, J. Chem. Soc. Farad. Trans 1, 72, (1976), 2150). According to these authors, the presence of a high potassium amount, particularly in the cancrinite cages, would be necessary to maintain at high temperature the cohesion of the crystalline lattice. It is possible to limit the destruction of highly exchanged NH.sub.4 KL forms during thermal treatments, by using thick beds (T. WEEKS and A. BOLTON, J. Phys. Chem. 79, (1975) 1924), however, the crystallinities, measured by DX of the solids obtained by this technique, are much lower than those of the initial solid.
U.S. Pat. No. 3,794,600 disclosing again a part of the process according to U.S. Pat. No. 3,375,065, claims a method for substantially completely removing potassium from L zeolite (K%&lt;0.35% by weight) without affecting the crystallinity of the solids. This method consists of roasting NH.sub.4 KL zeolite at temperatures lower than 600.degree. C. and performing exchanges in solutions of ammonium salts containing chromate ions. It is very important to note that in this document (U.S. Pat. No. 3,794,600) the crystallinity of the solids is not measured by X-ray diffraction but by benzene adsorption. Now, it has been proved (T. WEEKS and A. BOLTON, J. Phys. Chem. 79, (1975) 1924) that roasting of NH.sub.4 KL zeolite, exchanged at 80% (first step of U.S. Pat. No. 3,794,600), performed without particular care, leads to a severe degradation or even to a destruction of the DX crystallinity, whereas the adsorption capacity of the solid remains relatively very high. In these conditions, and inasmuch as the roasting conditions are not clearly defined in U.S. Pat. No. 3,794,600, it is very probably that the decationized solids obtained according to the procedures recommended in this document have a low DX crystallinity.
It thus appears that the present state of the art does not provide means for prepariing a decationized, dealuminated L zeolite of low mesh volume, stabilized and having a lattice of secondary pores. However, L zeolites having such characteristics are very useful since they make possible to prepare catalysts which are both active and selective in hydrocarbon conversion reactions.